Heretofore, a number of image forming methods have been known in which a photopolymerizing system is utilized. For example, there are a method wherein a photopolymerizable composition comprising an addition polymerizable compound containing an ethylenic double bond and a photopolymerization initiator, and optionally as an additional component an organic polymer binder, is prepared, this photopolymerizable composition is coated on a substrate to obtain a photosensitive material provided with a layer of the photopolymerizable composition, exposure of a desired image is conducted so that the exposed portion is polymerized and cured, and the unexposed portion is then dissolved and removed to form a cured relief image; a method wherein a layer of a photopolymerizable composition is formed between a pair of substrates, at least one of which is transparent, image exposure is conducted from the transparent substrate side so that a change in the bonding strength is created by the radiation, and then the substrate is peeled off to form an image; and a method wherein an image is formed by utilizing a change in the adhesion of a toner caused by radiation to a layer of a photopolymerizable composition. As the photopolymerization initiator of the photopolymerizable composition used in these methods, benzoin, benzoin alkyl ether, benzil ketal, benzophenone, anthraquinone, benzil or Michler's ketone has been employed. These photopolymerization initiators have an ability to initiate photopolymerization with light rays of short wavelength in the ultraviolet light range of at most 400 nm. However, their ability to initiate polymerization is poor with light rays in the visible light range of more than 400 nm. This poor ability has restricted the applicable range of photopolymerizable compositions containing them.
In recent years, with the progress of image-forming techniques, there has been a strong demand for photopolymers highly sensitive to light rays in the visible light range. Such photopolymers are photosensitive materials suitable for use in e.g. non-contact type projection exposure plate making or in laser plate making by means of a visible light laser. Among these techniques, a plate making system using an oscillation beam of 488 nm of an argon ion laser is considered to be one of the most prospective techniques for the future.
There have been some proposals for a photopolymerizable material containing a photopolymerization initiator system sensitive to light rays in the visible light range. For example, there have been proposed a system comprising a hexaarylbiimidazole, a radical-generating agent and a dye (Japanese Examined Patent Publication No. 37377/1970), a system comprising a hexaarylbiimidazole and a (p-dialkylaminobenzylidene)ketone (Japanese Unexamined Patent Publications No. 2528/1972 and No. 155292/1979), a system comprising a cyclic cis-.alpha.-dicarbonyl compound and a dye (Japanese Unexamined Patent Publication No. 84183/1973), a system comprising a substituted triazine and a merocyanine dye (Japanese Unexamined Patent Publication No. 151024/1979), a system comprising ketocumalin and an active agent (Japanese Unexamined Patent Publications No. 112681/1977, No. 15503/1983 and No. 88005/1985), a system comprising a substituted triazine and a sensitizer (Japanese Unexamined Patent Publications No. 29803/1983 and No. 40302/1983), a system comprising biimidazole, a styrene derivative and thiol (Japanese Unexamined Patent Publication No. 56403/1984), a system comprising an organic peroxide and a dye (Japanese Unexamined Patent Publications No. 140203/1984 and No. 189340/1984) and a s-triazine compound having a trihalomethyl group and a styryl group as a photopolymerization initiator effective with visible lights by itself (Japanese Examined Patent Publication No. 1281/1984).
These conventional techniques are certainly useful by visible light rays. However, they have a difficulty such that the sensitivity is not so high and is still inadequate from the practical viewpoint.